U.S. patent number 7,763,871 [Application Number 12/078,663] was granted by the patent office on 2010-07-27 for radiation source.
This patent grant is currently assigned to ASML Netherlands B.V.. Invention is credited to Vadim Yevgenyevich Banine, Vladimir Vitalevich Ivanov.
United States Patent |
7,763,871 |
Banine , et al. |
July 27, 2010 |
Radiation source
Abstract
A radiation source includes a chamber, a supply constructed and
arranged to supply a substance to the chamber at a location that
allows the substance to pass through an interaction point within
the chamber, a laser constructed and arranged to provide a laser
beam to the interaction point so that a radiation emitting plasma
is produced when the laser beam interacts with the substance at the
interaction point, and a conduit constructed and arranged to
deliver a buffer gas into the chamber. The conduit has an outlet
located adjacent to the interaction point.
Inventors: |
Banine; Vadim Yevgenyevich
(Helmond, NL), Ivanov; Vladimir Vitalevich (Moscow,
RU) |
Assignee: |
ASML Netherlands B.V.
(Veldhoven, NL)
|
Family
ID: |
41132399 |
Appl.
No.: |
12/078,663 |
Filed: |
April 2, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090250639 A1 |
Oct 8, 2009 |
|
Current U.S.
Class: |
250/504R;
250/493.1 |
Current CPC
Class: |
H05G
2/008 (20130101); G03F 7/70033 (20130101); H05G
2/003 (20130101); G03F 7/70916 (20130101); G03F
7/70175 (20130101) |
Current International
Class: |
A61N
5/06 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2003-8124 |
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Jan 2003 |
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JP |
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2005-197081 |
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Jul 2005 |
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JP |
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01/31678 |
|
May 2001 |
|
WO |
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2004/092693 |
|
Oct 2004 |
|
WO |
|
Other References
International Search Report and Written Opinion issued on Feb. 4,
2009 in International Application No. PCT/IB2008/002201. cited by
other.
|
Primary Examiner: Souw; Bernard E
Assistant Examiner: Smyth; Andrew
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman
LLP
Claims
What is claimed is:
1. A radiation source comprising: a chamber; a supply constructed
and arranged to supply a substance to the chamber at a location
that allows the substance to pass through an interaction point
within the chamber; a laser constructed and arranged to provide a
laser beam to the interaction point so that a radiation emitting
plasma is produced when the laser beam interacts with the substance
at the interaction point; a collector constructed and arranged to
collect radiation emitted from the plasma and focus the radiation
to a focal point; and a conduit constructed and arranged to deliver
a buffer gas into the chamber, the conduit having an outlet located
adjacent to the interaction point so that the outlet is located
closer to the interaction point than the collector.
2. The source of claim 1, wherein the outlet is located within the
outer boundary of a region within which heated buffer gas would be
continually present during operation of the source if buffer gas
were not supplied through the conduit.
3. The source of claim 1, wherein the outlet of the conduit is
located 15 cm or less from the interaction point.
4. The source of claim 3, wherein the outlet of the conduit is
located 10 cm or less from the interaction point.
5. The source of claim 1, wherein the outlet of the conduit is
located 3 cm or more from the interaction point.
6. The source of claim 1, wherein the conduit is located such that
it does not obscure radiation which would not otherwise be obscured
by some other component of the source.
7. The source of claim 1, wherein at least part of the conduit runs
alongside a gas cooler of the source.
8. The source of claim 1, wherein at least part of the conduit
passes through an aperture in the collector.
9. The source of claim 8, wherein at least part of the conduit
comprises two tubes, one of which is inside the other, an inner
tube being arranged such that the laser beam may pass along the
inner tube, and a channel between the two tubes being arranged to
allow the buffer gas to pass along the channel.
10. A method of generating radiation, comprising: introducing a
plasma generating substance into a chamber; directing a laser beam
at the plasma generating substance in order to produce a radiation
emitting plasma; collecting radiation emitted from the plasma with
a collector; and introducing buffer gas into the chamber at a
location adjacent to a point at which the laser beam and the plasma
generating substance interact, the location being closer to the
point at which the laser beam and the plasma generating substance
interact than the collector.
11. The method of claim 10, wherein the location at which the
buffer gas is introduced is within the outer boundary of a region
within which heated buffer gas would be continually present during
operation of the source if the buffer gas were not supplied to the
location adjacent the point at which the laser beam and the plasma
generating substance interact.
12. The method of claim 10, wherein the buffer gas is introduced
with a velocity of 100 m/s or greater.
13. The method of claim 10, wherein the buffer gas is introduced
with a velocity of 2000 m/s or less.
14. The method of claim 10, wherein the rate at which buffer gas is
introduced is sufficient to substantially remove heated buffer gas
from a region around the point at which the laser beam and the
plasma generating substance interact prior to a subsequent
interaction between the laser beam and the plasma generating
substance.
15. A lithographic apparatus comprising: a source of radiation
comprising a chamber; a supply constructed and arranged to supply a
substance to the chamber at a location that allows the substance to
pass through an interaction point within the chamber; a laser
constructed and arranged to provide a laser beam to the interaction
point so that a radiation emitting plasma is produced when the
laser beam interacts with the substance at the interaction point; a
collector constructed and arranged to collect radiation emitted
from the plasma and focus the radiation to a focal point; and a
conduit constructed and arranged to deliver a buffer gas into the
chamber, the conduit having an outlet located adjacent to the
interaction point so that the outlet is located closer to the
interaction point than the collector; an illumination system
configured to condition the radiation into a radiation beam; a
support structure configured to support a patterning device, the
patterning device being constructed and arranged to impart the
radiation beam with a pattern in its cross-section; a substrate
table configured to hold a substrate; and a projection system
configured to project the patterned radiation beam onto a target
portion of the substrate.
16. The lithographic apparatus of claim 15, wherein the outlet of
the conduit is located 15 cm or less from the interaction
point.
17. The lithographic apparatus of claim 16, wherein the outlet of
the conduit is located 10 cm or less from the interaction
point.
18. The lithographic apparatus of claim 15, wherein the outlet of
the conduit is located 3 cm or more from the interaction point.
Description
FIELD
The present invention relates to a radiation source, a method of
generating radiation, and to a lithographic apparatus which
includes the radiation source.
BACKGROUND
A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
In order to be able to project ever smaller structures onto
substrates, it has been proposed to use extreme ultraviolet
radiation having a wavelength within the range of 10-20 nm, for
example within the range of 13-14 nm. It has further been proposed
that radiation with a wavelength of less than 10 nm could be used,
for example 6.7 nm or 6.8 nm. In the context of lithography,
wavelengths of less than 10 nm are sometimes referred to as `beyond
EUV`.
Extreme ultraviolet radiation and beyond EUV radiation may be
produced using a plasma. The plasma may be created for example by
directing a laser at particles of a suitable material (e.g. tin),
or by directing a laser at a stream of a suitable gas (e.g. Sn
vapor, SnH.sub.4, or a mixture of Sn vapor and any gas with a small
nuclear charge (for example from H.sub.2 up to Ar)). The resulting
plasma emits extreme ultraviolet radiation (or beyond EUV
radiation), which may be collected and focused to a focal point
using a collector mirror.
In addition to extreme ultraviolet radiation (or beyond EUV
radiation), the plasma produces debris in the form of particles,
such as thermalized atoms, ions, nanoclusters, and/or
microparticles. The debris may cause damage to the collector mirror
(or other components). A buffer gas may be provided in the vicinity
of the plasma. The particles produced by the plasma collide with
molecules of the buffer gas, and thereby lose energy. In this way,
at least some of the particles may be slowed sufficiently that they
do not reach the collector mirror. Damage caused to the collector
mirror may thereby be reduced. However, even when buffer gas is
used, some particles may still reach the collector mirror and cause
damage to it.
It is desirable to improve the effectiveness of the buffer gas.
SUMMARY
According to an aspect of the invention there is provided a
radiation source comprising a chamber and a supply of a plasma
generating substance, the source having an interaction point at
which the plasma generating substance introduced into the chamber
may interact with a laser beam and thereby produce a radiation
emitting plasma, wherein the source further comprises a conduit
arranged to deliver a buffer gas into the chamber, the conduit
having an outlet which is adjacent to the interaction point.
According to an aspect of the invention, there is provided a
radiation source that includes a chamber, a supply constructed and
arranged to supply a substance to the chamber at a location that
allows the substance to pass through an interaction point within
the chamber, a laser constructed and arranged to provide a laser
beam to the interaction point so that a radiation emitting plasma
is produced when the laser beam interacts with the substance at the
interaction point, and a conduit constructed and arranged to
deliver a buffer gas into the chamber. The conduit has an outlet
located adjacent to the interaction point.
According to an aspect of the invention there is provided a method
of generating radiation comprising introducing a plasma generating
substance into a chamber and directing a laser beam at it in order
to produce a radiation emitting plasma, wherein the method further
comprises introducing buffer gas into the chamber at a location
which is adjacent to a point at which the laser beam and the plasma
generating substance interact.
According to an aspect of the invention, there is provided a method
of generating radiation. The method includes introducing a plasma
generating substance into a chamber; directing a laser beam at the
plasma generating substance in order to produce a radiation
emitting plasma, and introducing buffer gas into the chamber at a
location adjacent to a point at which the laser beam and the plasma
generating substance interact.
According to an aspect of the invention there is provided a
lithographic apparatus comprising a source of radiation, an
illumination system for conditioning the radiation, a support
structure for supporting a patterning device, the patterning device
serving to impart the radiation beam with a pattern in its
cross-section, a substrate table for holding a substrate, and a
projection system for projecting the patterned radiation beam onto
a target portion of the substrate, wherein the radiation source
comprises a chamber and a supply of a plasma generating substance,
the source having an interaction point at which the plasma
generating substance introduced into the chamber may interact with
a laser beam and thereby produce a radiation emitting plasma, the
source further comprising a conduit arranged to deliver a buffer
gas into the chamber, and the conduit having an outlet which is
adjacent to the interaction point.
According to an aspect of the invention, there is provided a
lithographic apparatus that includes a source of radiation. The
source of radiation includes a chamber, a supply constructed and
arranged to supply a substance to the chamber at a location that
allows the substance to pass through an interaction point within
the chamber, a laser constructed and arranged to provide a laser
beam to the interaction point so that a radiation emitting plasma
is produced when the laser beam interacts with the substance at the
interaction point, and a conduit constructed and arranged to
deliver a buffer gas into the chamber. The conduit has an outlet
located adjacent to the interaction point. The lithographic
apparatus also includes an illumination system configured to
condition the radiation into a radiation beam, and a support
structure configured to support a patterning device. The patterning
device is constructed and arranged to impart the radiation beam
with a pattern in its cross-section. The lithographic apparatus
further includes a substrate table configured to hold a substrate,
and a projection system configured to project the patterned
radiation beam onto a target portion of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying schematic drawings
in which corresponding reference symbols indicate corresponding
parts, and in which:
FIG. 1 depicts a lithographic apparatus according to an embodiment
of the invention;
FIGS. 2a and 2b depict a radiation source according to an
embodiment of the invention; and
FIG. 3 depicts a radiation source according to an alternative
embodiment of the invention.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a lithographic apparatus according to
one embodiment of the invention. The apparatus comprises: an
illumination system (illuminator) IL configured to condition a
radiation beam B (e.g. EUV radiation or beyond EUV radiation); a
support structure (e.g. a mask table) MT constructed to support a
patterning device (e.g. a mask) MA and connected to a first
positioner PM configured to accurately position the patterning
device in accordance with certain parameters; a substrate table
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioner PW
configured to accurately position the substrate in accordance with
certain parameters; and a projection system (e.g. a refractive
projection lens system) PS configured to project a pattern imparted
to the radiation beam B by patterning device MA onto a target
portion C (e.g. comprising one or more dies) of the substrate
W.
The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
The support structure supports, i.e. bears the weight of, the
patterning device. It holds the patterning device in a manner that
depends on the orientation of the patterning device, the design of
the lithographic apparatus, and other conditions, such as for
example whether or not the patterning device is held in a vacuum
environment. The support structure can use mechanical, vacuum,
electrostatic or other clamping techniques to hold the patterning
device. The support structure may be a frame or a table, for
example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
Examples of patterning devices include masks and programmable
mirror arrays. Masks are well known in lithography, and typically
in an EUV or beyond EUV lithographic apparatus would be reflective.
An example of a programmable mirror array employs a matrix
arrangement of small mirrors, each of which can be individually
tilted so as to reflect an incoming radiation beam in different
directions. The tilted mirrors impart a pattern in a radiation beam
which is reflected by the mirror matrix.
The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system. Usually,
in an EUV or beyond EUV lithographic apparatus the optical elements
will be reflective. However, other types of optical element may be
used. The optical elements may be in a vacuum. Any use of the term
"projection lens" herein may be considered as synonymous with the
more general term "projection system".
As here depicted, the apparatus is of a reflective type (e.g.
employing a reflective mask).
The lithographic apparatus may be of a type having two (dual stage)
or more substrate tables (and/or two or more mask tables). In such
"multiple stage" machines the additional tables may be used in
parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for
exposure.
Referring to FIG. 1, the illuminator IL receives a radiation beam
from a radiation source SO. The source and the lithographic
apparatus may be separate entities. In such cases, the source is
not considered to form part of the lithographic apparatus and the
radiation beam is passed from the source SO to the illuminator IL
with the aid of a beam delivery system comprising, for example,
suitable directing mirrors and/or a beam expander. In other cases
the source may be an integral part of the lithographic apparatus.
The source SO and the illuminator IL, together with the beam
delivery system if required, may be referred to as a radiation
system.
The illuminator IL may comprise an adjuster for adjusting the
angular intensity distribution of the radiation beam. Generally, at
least the outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in a pupil plane of the illuminator can be adjusted.
In addition, the illuminator IL may comprise various other
components, such as an integrator and a condenser. The illuminator
may be used to condition the radiation beam B to have a desired
uniformity and intensity distribution in its cross-section.
The radiation beam B is incident on the patterning device (e.g.,
mask MA), which is held on the support structure (e.g., mask table
MT), and is patterned by the patterning device. Having been
reflected by the mask MA, the radiation beam B passes through the
projection system PS, which focuses the beam onto a target portion
C of the substrate W. With the aid of the second positioner PW and
position sensor IF2 (e.g. an interferometric device, linear encoder
or capacitive sensor), the substrate table WT can be moved
accurately, e.g. so as to position different target portions C in
the path of the radiation beam B. Similarly, the first positioner
PM and another position sensor IF1 can be used to accurately
position the mask MA with respect to the path of the radiation beam
B, e.g. after mechanical retrieval from a mask library, or during a
scan. In general, movement of the mask table MT may be realized
with the aid of a long-stroke module (coarse positioning) and a
short-stroke module (fine positioning), which form part of the
first positioner PM. Similarly, movement of the substrate table WT
may be realized using a long-stroke module and a short-stroke
module, which form part of the second positioner PW. In the case of
a stepper (as opposed to a scanner) the mask table MT may be
connected to a short-stroke actuator only, or may be fixed. Mask MA
and substrate W may be aligned using mask alignment marks M1, M2
and substrate alignment marks P1, P2. Although the substrate
alignment marks as illustrated occupy dedicated target portions,
they may be located in spaces between target portions (these are
known as scribe-lane alignment marks). Similarly, in situations in
which more than one die is provided on the mask MA, the mask
alignment marks may be located between the dies.
The depicted apparatus could be used in at least one of the
following modes:
1. In step mode, the mask table MT and the substrate table WT are
kept essentially stationary, while an entire pattern imparted to
the radiation beam is projected onto a target portion C at one time
(i.e. a single static exposure). The substrate table WT is then
shifted in the X and/or Y direction so that a different target
portion C can be exposed. In step mode, the maximum size of the
exposure field limits the size of the target portion C imaged in a
single static exposure.
2. In scan mode, the mask table MT and the substrate table WT are
scanned synchronously while a pattern imparted to the radiation
beam is projected onto a target portion C (i.e. a single dynamic
exposure). The velocity and direction of the substrate table WT
relative to the mask table MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning device, and the
substrate table WT is moved or scanned while a pattern imparted to
the radiation beam is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning device is updated as required after each
movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
Combinations and/or variations on the above described modes of use
or entirely different modes of use may also be employed.
FIGS. 2a and 2b show schematically a source SO according to an
embodiment of the invention. FIG. 2a shows the source SO in cross
section viewed from one side, and FIG. 2b shows the source in cross
section viewed from above.
The source SO comprises a chamber 1. The chamber 1 is defined by
walls 2 and a collector mirror 3. The collector mirror 3 has a
reflective surface which is reflective at extreme ultraviolet
radiation wavelengths.
A supply 4 is arranged to supply droplets of material (for example
tin) into the chamber 1. A collector 5 is located beneath the
supply 4 at the bottom of the chamber 1, and is arranged to collect
material which has passed through the chamber 1.
The collector mirror 3 is arranged to focus radiation to a focal
point FP, from where the radiation may pass into the illuminator IL
of the lithographic apparatus (see FIG. 1). A laser 6 is used to
generate a beam of radiation 7 which is directed into the chamber 1
via an aperture 8. The aperture 8 may for example comprise a window
which is transmissive at the wavelength of the laser beam 7. A beam
dump 9 is located within the chamber 1, and is positioned such that
any portion of the laser beam 7 which does not interact with
material provided by the material supply 4 is incident upon (and
absorbed by) the beam dump. Gas coolers 10 extend into the chamber
1 from side walls of the chamber.
A buffer gas supply comprises a conduit 11 which extends into the
chamber 1 from a side wall of the chamber, and has an outlet 12
which delivers buffer gas adjacent to an interaction point 13 at
which the laser beam 7 is incident upon material supplied from the
material supply 4.
In use, the chamber 1 is filled with a suitable buffer gas (for
example hydrogen). The laser 6 generates a laser beam 7, which
passes through the aperture 8 in the collector mirror 3 and into
the chamber 1. The material supply 4 produces a droplet of material
which falls downwards through the chamber 1 towards the collector
5. When the droplet of material passes through the interaction
point 13, the interaction of the laser beam 7 and the droplet of
material causes at least some of the material to be converted into
a plasma. The plasma emits extreme ultraviolet radiation, which is
collected by the collector mirror 3 and focused to the focal point
FP. The extreme ultraviolet radiation passes from the focal point
FP into the illuminator IL of the lithographic apparatus (see FIG.
1).
Parts of the droplet of material which do not interact with the
laser beam 7 continue to fall through the chamber 1 and are
collected by the collector 5.
The plasma generated by the interaction of the laser beam 7 and the
droplet of material may include particles which would cause damage
to the collector mirror 3. The buffer gas present in the chamber 1
is intended to slow down the particles so that they do not reach
the collector mirror 3. However, the violence of the interaction
between the laser beam 7 and the tin particle at the interaction
point 13 is such that the buffer gas is heated and pushed away from
the interaction point when the laser beam interacts with the
droplet of material. This may cause the buffer gas in a region
around the interaction point to have a higher temperature and a
lower density.
In a conventional extreme ultraviolet radiation source (in which
the buffer gas is introduced from a sidewall of the chamber), some
time will elapse before the heated buffer gas moves away from the
region around the interaction point 13 (the heated buffer gas may
for example move towards the gas coolers 10). The time taken for
the heated buffer gas to move away from the region around the
interaction point 13 may for example be of the order of tens of
milliseconds. The time between delivery of successive droplets of
material to the interaction point 13 may be significantly shorter
than this, for example 10-20 microseconds. This means that the
heated buffer gas may remain present in the region around the
interaction point 13 during the generation of successive pulses of
EUV radiation.
The region around the interaction point 13 which is occupied by the
heated buffer gas may comprise a significant proportion of the
volume between the interaction point 13 and the collector mirror 3.
The heated buffer gas in this region has a lower density than gas
which has not been heated, and a result there are less interactions
between the particles of the plasma and the buffer gas.
Consequently, it is more likely that particles may reach the
collector mirror 3. When this occurs, damage may be caused to the
collector mirror 3.
There is an additional effect which may contribute to the problem
described above. Many of the fast ions generated at the interaction
point 13 are moving in the direction of the collector mirror 3.
When these fast ions are stopped by the buffer gas, they transfer
their momentum to the buffer gas, thereby causing the buffer gas to
flow in the direction of the collector mirror 3. This further
reduces the density of the buffer gas in the region around the
interaction point 13.
The above problem may be solved or reduced in magnitude by the
conduit 11 shown in FIG. 2. The conduit 11 has an outlet 12 which
is located adjacent to the interaction point 13, and thereby
delivers unheated buffer gas adjacent to the interaction point 13.
Thus, instead of unheated buffer gas flowing into the region around
the interaction point 13 only after heated buffer gas has moved
away from that region, the outlet 12 of the conduit 11 immediately
and directly delivers unheated buffer gas into the region around
the interaction point 13. Consequently, by the time the next
droplet of material has reached the interaction point 13, newly
delivered buffer gas will be present in the region around the
interaction point 13.
This newly delivered buffer gas is unheated and is therefore more
dense than heated buffer gas. The buffer gas is therefore more
effective. The embodiment of the invention therefore may provide
improved protection of the collector mirror 3 from particles
generated during plasma formation. It may therefore allow the
collector mirror 3 to have a longer lifetime before cleaning and/or
replacement than may otherwise be the case.
The buffer gas may be delivered with a high velocity (for example
100-2000 m/s). This may provide the advantage that it quickly
pushes away heated buffer gas from the region around the
interaction point 13. The buffer gas may be delivered in a
supersonic gas jet which is directed at or adjacent to the
interaction point 13. The supersonic gas jet may have the advantage
that the density of buffer gas within the jet may be substantially
larger than the mean density of buffer gas in the chamber, thereby
providing an increased interaction of fast ions with the buffer gas
adjacent to the interaction point 13.
Since the conduit 11 is introducing buffer gas into the chamber 1,
one or more vents (not shown) may be used to carry buffer gas from
the chamber 1, and thereby regulate the pressure of buffer gas
within the chamber. The gas coolers 10 regulate the temperature of
the buffer gas.
The conduit 11 is provided at a location which is selected such
that extreme ultraviolet radiation which is obscured by the conduit
11 would have been obscured by other elements of the apparatus if
the conduit 11 were not present. Thus, the conduit 11 is located in
front of a gas cooler 10 which would obscure the EUV radiation
irrespective of whether or not the conduit 11 is present. The
conduit 11 is vertically displaced with respect to the laser beam
7, so that the laser beam does not pass into the conduit 11, but
instead travels next to it and is incident upon the beam dump
9.
As has previously been mentioned, the outlet of the conduit 11 is
adjacent to the interaction point 13. The outlet of the conduit 11
may be within the outer boundary of a region within which heated
buffer gas would be continually present during operation of the EUV
source if buffer gas were not supplied through the conduit 11.
The distance between the outlet 12 of the conduit 11 and the
interaction point 13 may be selected by considering the following:
the closer the outlet 12 is to the interaction point 13, the more
effective the delivery of unheated buffer gas to the region around
the interaction point 13. However, the closer the outlet 12 is to
the interaction point 13, the more the conduit 11 is likely to
suffer from sputtering of ions against the conduit. In one example,
the outlet 12 may be about 15 cm or less from the interaction
point, and may be about 10 cm or less from the interaction point.
The outlet may be about 3 cm or more from the interaction point.
The distance between the interaction point 13 and the collector
mirror 3 may be about 20 cm.
The rate at which buffer gas is provided through the outlet 12 may
be sufficient to substantially remove heated buffer gas from the
region around the interaction point 13. The rate may be sufficient
to achieve this before the next laser and material droplet
interaction. The rate at which buffer gas should be provided
through the outlet 12 in order to achieve this may be calculated
based upon the volume of buffer gas that is heated by a laser and
material droplet interaction, and the frequency at which laser and
material droplet interactions take place (i.e. the frequency of the
EUV source).
An embodiment of the invention is shown schematically in FIG. 3.
FIG. 3 shows a source SO viewed from one side. The majority of
elements of the source SO shown in FIG. 3 correspond with those
shown in FIGS. 2a and 2b, and are not described again here.
However, the conduit 11 of FIG. 2b is not present in FIG. 3.
Instead, a conduit 21 passes through the aperture 8 in the
collector mirror 3, and travels parallel to the laser beam 7. The
conduit 21 is provided with an outlet 22 which is adjacent to the
interaction point 13. The conduit 21 is used to introduce buffer
gas adjacent to the interaction point 13 in an equivalent manner to
that described above in relation to FIG. 2. The conduit 21 is
positioned such that, while it may obscure some EUV radiation
generated by the plasma in the chamber 1, the amount of EUV
radiation which is obscured is relatively small (for example, only
the cross-section of the conduit obscures the EUV radiation rather
than its length). The distance between the outlet 22 and the
interaction point 13 may be selected using the criteria that were
described further above in relation to FIG. 2.
A potential advantage of the embodiment shown in FIG. 3 is that the
flow of buffer gas provided by the conduit is away from the
collector mirror 3 rather than towards it (thereby helping to push
heated buffer gas away from the collector mirror 3).
In a modified version of the embodiment shown in FIG. 3, the
conduit may consist of two tubes, one of which is inside the other.
The laser beam may be arranged to pass along the inner of the two
tubes, and the buffer gas may be arranged to pass along a channel
formed between the two tubes. Where this is the case, the corner
shown in FIG. 3 may be absent from the inner of the two tubes, in
order to allow the laser beam to travel unimpeded from the laser to
the interaction point.
Although conduits 11, 21 having different positions and
configurations have been shown in FIGS. 2b and 3, other conduit
positions and configurations may be used. It is desirable that the
conduit position and configuration is such that it does not obscure
any EUV radiation which would not otherwise be obscured by some
other component of the source SO. In some instances, this may not
be achievable or it may be desired to provide the conduit in some
location wherein the conduit does indeed obscure some EUV
radiation. Where this is the case, it is desirable to minimize the
amount of EUV radiation which is obscured by the conduits where
possible. Appropriate locations and configurations for the conduit
will depend upon the particular arrangement of the source within
which the conduit is provided. More than one conduit may be
provided (for example the conduits shown in FIGS. 2b and 3 may both
be provided in a single EUV source).
Although the above description has referred to the use of hydrogen
as the buffer gas, other suitable gases may be used.
Although the above description has referred to the droplets of
material being tin, other suitable materials may be used.
The invention is not limited to radiation sources which use
droplets of material. An embodiment of the invention and may for
example generate plasma from a gas rather than from droplets of
material. Suitable gases include Sn vapor, SnH.sub.4, or a mixture
of Sn vapor and any gas with a small nuclear charge (for example
from H.sub.2 up to Ar). Droplets of material or gases may be
considered to be examples of a plasma generating substance.
The wavelength of the EUV radiation referred to in the above
description may for example be within the range of 10-20 nm, for
example within the range of 13-14 nm.
Although the above description of embodiments of the invention
relates to a radiation source which generates EUV radiation, the
invention may also be embodied in a radiation source which
generates `beyond EUV` radiation, that is radiation with a
wavelength of less than 10 nm. Beyond EUV radiation may for example
have a wavelength of 6.7 nm or 6.8 nm. A radiation source which
generates beyond EUV radiation may operate in the same manner as
the radiation sources described above.
In the above description the term `unheated buffer gas` is intended
to mean buffer gas which is delivered from the outlet 12, 22 after
an interaction between the laser beam and the plasma generating
substance (and before the next interaction between the laser beam
and the plasma generating substance).
The description above is intended to be illustrative, not limiting.
Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
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